Patent GB-1034629-A (GB)
Title: Improvements relating to television distribution systems
IPC: H04N7/10
Filed: 1965-04-07
Abstract: 1,034,629. Wired television distribution systems. OFFICE DE RADIODIFFUSION TELEVISION FRANCAISE. April 7, 1965 [April 7, 1964], No. 14799/65. Heading H4R. Wired distribution system for television signals includes central equipment 1 to 4, Fig. 1, comprising, for each programme to be distributed, a channel for demodulating the television signals of the programme and modulating the vision signal by a distribution carrier whose frequency is equal to the difference between the sound and vision frequencies of the programme. The upper sideband of the resulting modulated signal is partially suppressed and added to the demodulated sound signal for transmission over the distribution network which includes separate wires for each programme. At the receiver the signals are modulated by a carrier equal in frequency to the sound carrier of a channel of a standard similar to that of the programme being received, to produce a double sideband signal, one sideband of which has the same characteristics as a broadcast transmission in the same frequency channel, and which can therefore be received by a standard television receiver 11. The standard receiver circuits are sufficient to suppress the unwanted sideband of the double sideband signal. The embodiment described with respect to Fig. 1 is adapted to French television standards and comprises a system for distributing an 819 line signal from band III and three 625 line signals from bands IV and V, one of which is a colour signal compatible with the other signals. In each case the received signal is demodulated in a demodulator, e.g. 102, and the video signal remodulated by a carrier, whose frequency corresponds to the difference between the sound and video carriers, in a modulator, e.g. 106. If there is likely to be cross-talk between the individual transmissions the exact frequencies of the distribution carriers may be chosen, as illustrated in Fig. 1 for channels 2, 3 and 4, to differ by a fraction of the line frequency, to eliminate patterning, while the colour television distribution carrier is synchronized to be an exact multiple of the line frequency, to provide optimum protection of the chrominance subcarrier. The modulated vision signal is then fed through filter 107 to partially suppressed the upper sideband before the demodulated sound signal is added in adder 108. The composite signal is then applied to a line via phase and frequency characteristic compensating networks and amplifier 109 and 110. The line may be provided with further equalizers 111 and 112 before it is fed to the subscriber's equipment via junction boxes and equalizers 120. At the subscriber's equipment the signal is modulated by a local carrier to a frequency corresponding to the normal received frequency of signals of the appropriate standard, so that they may be reproduced by a normal television receiver 111. As shown, the signals of similar standard may be selected by a switch 15 before being fed to a single modulator fed by a local oscillator 16, alternatively the incoming signals could be fed to individual modulators fed with different oscillator frequencies so that the programme could be selected by the normal receiver channel selector.